In the development of containers for the retention of liquid under pressure, for example, beer, carbonated beverages and the like, it is desirable to provide the container with a "pressure bottom" configuration. Such pressure bottom configurations are arcuate and may be either concave or convex as set forth in my prior application Ser. No. 225,449, now U.S. Pat. No. 3,843,005. These arcuate bottom configurations avoid a sharp contour transition portion located at the "heel" of a conventional flat-bottom container, and, as a result, enhanced bottom strength in the container is obtained. This greater bottom strength is an advantage which may be obtained in several ways. First, the arcuate design itself is better adapted to serve as a pressure vessel. Secondly, by not moving the plastic material during blowing into sharp bottom areas or heels of the container, the unavoidable thinning of the plastic material at the heel does not occur.
The formation of blown plastic containers having arcuate pressure bottom configurations is quite difficult because the "blow-up" of a cylindrical parison to the intricate, concavo-convex configuration results in the thinning of the material in the bottom area. Additionally, the bottom region of the preform or parison is generally hotter than the remainder of the parison, and it is difficult to biaxially orient the bottoms of blown containers. In my copending application, Ser. No. 103,624, now U.S. Pat. No. 3,767,747 there is described a method and apparatus for obtaining a vessel or container having enhanced physical strength because of the molecular orientation (stretch orientation) of the thermoplastic material during manufacture. However, prior to my present invention, there was no satisfactory method or apparatus for providing blown plastic containers having (1) pressure bottoms of uniform thickness and (2) pressure bottoms which are biaxially oriented.
Furthermore, the prior art cannot provide such pressure bottoms, where the container is made initially from a freely extruded parison. because the accumulation of material at the pinch-off exaggerates both the material distribution problem and the temperature differential problem.